FIG. 9 is a schematic view showing a conventional image printing apparatus using a color inkjet printing method.
In the image printing apparatus of FIG. 9, a motor 103 is driven in printing (drawing) an image on a printing medium 140 on a platen 106. A carriage 102 having printheads 120 to 123 is moved by a driving belt 109 to the position of a home position sensor 108. While the carriage 102 is moved along a forward pass indicated by an arrow X1 in the scanning direction, inks in black K, cyan C, magenta M, and yellow Y are discharged at a predetermined position from the printheads 120, 121, 122, and 123 in accordance with input image data, printing a predetermined image 133.
After the image 133 is printed by a predetermined length represented by 134 in FIG. 9, movement of the carriage 102 along the forward pass X1 of the scanning direction stops. While the carriage 102 is moved along a return pass indicated by an arrow X2 opposite to the forward pass in the scanning direction, the carriage 102 is returned to the start position (position of the home position sensor 108) for printing/scanning of the next image. While the carriage 102 is moved along the return pass, a feed roller 110 is rotated by a feed motor 107 to convey the printing medium 140 in a subscanning direction (direction indicated by an arrow Y) perpendicular to the main scanning direction by a length corresponding to the width 134 by which the image is printed by the printheads 120 to 123.
As described above, an image is printed on a printing medium while moving the carriage 102 in the main scanning direction, and the printing medium is conveyed in the subscanning direction by the width 134 of one band. This operation is repeated to complete printing of a color image.
Image printing operation in only the forward pass in the main scanning direction has been exemplified. Bi-directional image printing operation in both the forward and return passes in the main scanning direction is also possible. In this case, an image is printed in the forward pass, and the printing medium 140 is conveyed in the subscanning direction by a length corresponding to the width 134 of one band by which the image is printed by the printheads 120 to 123. After that, image printing is executed in the return pass in the main scanning direction, printing an image in both the forward and return passes. In FIG. 9, reference numerals 100 and 101 denote second feed rollers; and 111, a medium detection sensor.
The discharge timings of ink from the nozzles of the printheads 120 to 123 are generated by using an output signal from a linear encoder to be described later as a reference. The position of each printhead is detected by the linear encoder, and the linear encoder can detect the position at a precision corresponding to a necessary resolution (e.g., 1,200 dpi). In an image printing apparatus having such linear encoder, the image printing resolution and the precision of the image printing position are determined by a position detection signal output from the linear encoder.
The image printing apparatus realizes multicolor image printing by superposing black (K), cyan (C), magenta (M), and yellow (Y) inks discharged from the printheads 120, 121, 122, and 123 for image data (printing data) corresponding to the same pixel on the basis of position information from the linear encoder and the relative positions of the printheads 120, 121, 122, and 123. Hence, position information from a linear encoder 130 greatly influences the image quality.
At present, linear encoders used in such image printing apparatuses are generally a magnetic linear encoder, and an optical linear encoder 130 shown in FIG. 9. For example, the magnetic linear encoder is comprised of a metal linear scale plate formed by many magnetization portions in the scale unit, and a magnetic sensor which is attached onto the carriage 102 and detects magnetism at the magnetization portions of the linear scale plate.
As shown in FIG. 9, the optical linear encoder 130 is comprised of a band-like scale 131 which has a graduated grid and is formed by alternately printing a light-reflecting portion and non-reflecting portion on low-expansion-coefficient glass in the scale unit, and a sensor 132 which irradiates the scale 131 with light and receives light reflected by the scale 131. The sensor 132 is generally a device (light-projecting/receiving device) constituted by a light-projecting portion formed from an LED or laser source attached onto the carriage 102, and a light-receiving portion which is formed from a photodiode or phototransistor.
Either magnetic or optical linear encoder uses a home position as a reference position. Read pulse signals which are output from the sensor in the linear scale unit in response to movement of the carriage 102 are counted up/down by an encoder counter. The count value is read to obtain position information of the carriage 102 (e.g., Japanese Patent Laid-Open No. 2000-168151).
The image printing apparatus can print an image by a 300-dpi system, i.e., at resolutions of 1,200 dpi, 600 dpi, and 300 dpi for a linear encoder resolving power of 1,200 dpi, but cannot print an image at resolutions of 1,440 dpi and 720 dpi.
In general, the resolution of the image printing apparatus belongs to two systems: a 300-dpi system having resolutions of 300 dpi, 600 dpi, 1,200 dpi, . . . and a 360-dpi system having resolutions of 360 dpi, 720 dpi, 1,440 dpi, . . . . Most of the nozzle intervals of printheads used for image printing are formed in accordance with either system.
However, some recent image printing apparatuses print an image at an arbitrary resolution other than the 300- and 360-dpi systems, like an image printing apparatus which forms a liquid crystal filter. In the image printing apparatus which forms a liquid crystal filter, the landing precision of an ink dot discharged onto a printing medium must be as high as about several μm, and the cost of the image printing apparatus becomes high. Demands have therefore arisen for an image printing apparatus which can print an image at various resolutions such as the 300- and 360-dpi systems.
On the other hand, either type of encoder described above suffers a read position error depending on the component/assembly precision and scale patterning precision in manufacturing an encoder, and further a read position error caused by thermal expansion of the scale itself. These position errors are negligible in a general inkjet printer. In the image printing (drawing) apparatus for manufacturing a liquid crystal filter, the liquid crystal filter pattern is dense, and ink must be landed on a target position at a high precision. To realize this, the read position error of the encoder depending on the component/assembly precision and scale patterning precision in manufacturing an encoder must fall within the allowable range. A feed error depending on the pitching, yawing, and straightness of the carriage and printing medium moving means must be corrected to make the ink landing position error fall within the allowable range.